Not applicable.
The present invention relates to improved calenders for calendering a web.
During the 1990""s a new calendering technique was introduced: the extended soft nip or the LN-calender (i.e., Long Nip) where the advantages of soft calendering were further developed. Compared to conventional soft nips, the local stress concentrations in the calender nip are substantially reduced with the extended soft nip. The required smoothing of the paper surface can therefore be obtained with minor or no increase of the local variations of the surface properties.
The LN-concept has been tested in different configurations; the shoe-belt and the roll-belt, see FIGS. 1A, 1B, and 1C.
The shoe-belt configuration 10 as shown in FIG. 1A consists of a stationary hydraulically loaded concave press shoe 12 (as shown in FIG. 1C) and an endless polymer belt 14. To prevent the friction heat developed between the stationary press shoe and the mobile polymer belt from becoming too high, an intermediate layer in the form of an oil film 16, which dissipates the pressure force, is used as shown in FIG. 1C. The length and shape of the press shoe are the dominating factors determining the nip length.
The roll-belt configuration 18, shown in FIG. 1B, is the second type of extended soft calender nip used. For this configuration, a rotary steel roll 20 instead of a stationary press shoe supports the polymer belt 22. The extended nip length is determined mainly by the belt thickness and the compressive deformation behavior of the polymer belt, which is significantly more deformable than a conventional backing roll cover. A roll that stretches the belt and an alignment roll that controls the CD-position of the belt are other necessary components of this configuration. The static nip length is estimated to be about 20-35 mm with the roll-belt configuration.
For some years both configurations were promoted. However, nowadays only the shoe-belt configuration 10 is marketed. The reason for this is due to difficulties with the scaling up of the roll-belt configuration 18, for example difficulties in controlling the cross machine direction alignment of the polymer belt 22. Nevertheless, the roll-belt configuration 18 has its advantages compared to the shoe-belt configuration, and these advantages are listed below.
Due to the fact that a rotary steel roll is used, instead of a lubricated stationary press shoe to support the polymer belt, a higher maximum compressive stress can be applied. This is beneficial when aiming for higher degrees of surface deformation of the paper products.
The construction of the shoe-belt configuration is simplified (and made possibly more cost effective) by avoiding the shoe press technology.
The energy consumption is most likely lower compared to the shoe-belt configuration since the nip is obtained without a stationary support, thus the friction, which consumes power, will be lower.
The calender of this invention has a heated first roll, mounted for rotation in a calender frame, which is opposed to and forms a nip with a soft surface counter roll system. The soft surface counter roll system is comprised of a second roll which has an outer shell. The second roll is mounted for rotation to the calender frame. An endless polyurethane belt or blanket having an inside diameter approximately 10 to 20 mm larger than the outside diameter of the second roll shell is mounted about the second roll. The outer circumferential edges of the polyurethane belt are held by clamping rings to bearings which are circumferentially mounted about the second roll on the roll gables. A small quantity of oil may be placed between the surface of the second roller shell and the polyurethane belt.
It is an object of the present invention to provide a soft nip calender of simpler design.
It is a further object of the present invention to provide a soft nip calender which provides greater maximum compressive forces.
It is another object of the present invention to provide a soft nip calender which is more energy-efficient.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.